1.
dropping formalities; setting aside rank; party where everyone is equal
A gathering where the usual hierarchical etiquette between seniors and juniors is temporarily suspended, allowing everyone to relax and enjoy themselves as equals. Common at company drinking parties.
The department head said 'no formalities tonight,' so everyone started drinking in a relaxed manner.
Usage
無礼講 literally means "no-rudeness lecture" — a gathering where rudeness (breaking rank) is permitted. It is commonly declared at company 飲み会 (drinking parties) and 忘年会 (year-end parties).
Important cultural note
In practice, 無礼講 is often a polite fiction. While the boss may declare the gathering 無礼講, employees are still expected to maintain a degree of respect. Truly dropping all formality with superiors can still cause problems.